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Abstract

In this paper, we establish two new regularity criteria for the 3D magneto-micropolar
fluids in terms of one directional derivative of the velocity or of the pressure and
the magnetic field.

MSC:
35Q35, 76W05, 35B65.

Keywords:

magneto-micropolar fluid equations; regularity criteria

1 Introduction

In this paper, we consider the Cauchy problem of the 3D incompressible magneto-micropolar
fluid equations

(1.1)

where u is the fluid velocity, w is the micro-rotational velocity, b is the magnetic field and π is the pressure. Equations (1.1) describe the motion of a micropolar fluid which
is moving in the presence of a magnetic field (see [1]). The positive parameters μ, χ, γ, κ and ν in (1.1) are associated with the properties of the materials: μ is the kinematic viscosity, χ is the vortex viscosity, ν and κ are the spin viscosities and is the magnetic Reynolds number.

Recently, Yuan [2] investigated the local existence and uniqueness of the strong solutions to the magneto-micropolar
fluid equations (1.1) (see also [3-6] for the bounded domain cases). Thus, the further problem at the center of the mathematical
theory concerning equations (1.1) is whether or not it has a global in time smooth
solution for any prescribed smooth initial data, which is still a challenging open
problem. In the absence of a global well-posedness theory, the development of regularity
criteria is of major importance for both theoretical and practical purposes. We would
like to recall some related results in this direction.

Note that if the micro-rotation effects and the magnetic filed are not taken into
account, i.e., , equations (1.1) reduce to the classical Navier-Stokes equations. The global regularity
issue has been thoroughly investigated for the 3D Navier-Stokes equations and many
important regularity criteria have been established (see [7-16] and the references therein). In particular, the first well-known regularity criterion
is due to Serrin [14]: if the Leray-Hopf weak solution u of the 3D Navier-Stokes equations satisfies

then u is regular on . Beirao da Veiga [8] and Penel and Pokorny [13] established another regularity criteria by replacing the above conditions with the
following ones:

or

More recently, Cao and Titi [17] established a regularity criterion in terms of only one of the nine components of
the gradient of a velocity field, that is, the solution u is regular on if

where and , or

This result on is stronger than a similar result of Zhou and Pokorny [18] in the sense of allowing for much smaller values of p. These regularity criteria are of physical relevance since experimental measurements
are usually obtained for quantities of the form . The regularity criterion by imposing the growth conditions on the pressure field
are also examined by, for example, Berselli and Galdi [9], Chae and Lee [10] and Zhou [15,16], i.e., if

or

then the solution u is regular on (see also [14,17] for the Besov spaces cases). For the 3D Navier-Stokes equations with boundary conditions,
Cao and Titi first introduced a regularity criterion in terms of only one component
of the pressure gradient based on the breakthrough of the global regularity of the
3D primitive equations [19]. Recently, Cao and Titi [20] established a similar regularity criterion for the Cauchy problem of the 3D Navier-Stokes
equations, that is, the solution u is regular on if

When the micro-rotation effects are neglected, i.e., , equations (1.1) become the usual magnetohydrodynamic (MHD) equations. Some of the
regularity criteria established for the Navier-Stokes equations can be extended to
the 3D MHD equations by making assumptions on both u and b (see [21,22]). Moreover, He and Xin [23,24] showed that the velocity field u plays a dominant role in the regularity issue and derived a criterion in terms of
the velocity field u alone (see also [25,26] for the Besov spaces cases). Recently, Cao and Wu [27] further proved that if

or

then is regular on . More recently, Liu, Zhao and Cui [28] have adapted the method of [27] to establish a similar regularity criterion for the 3D nematic liquid crystal flow.

If we ignore the magnetic filed, i.e., , equations (1.1) reduce to the micropolar fluid equations. The theory of micropolar
fluid has attracted more and more scholars’ attention in recent years. In particular,
Dong, Jia and Chen [29] recently established a regularity criterion via the pressure field, which says that
if

For the full magneto-micropolar fluid equations (1.1), Yuan [32] recently showed that the solution is regular on if

(1.2)

or

(1.3)

For other regularity criteria of equations (1.1), we refer to Gala [33], Geng, Chen and Gala [34], Wang, Hu and Wang [35], Yuan [2] and Zhang, Yao and Wang [36].

In this paper, we establish two new regularity criteria for the 3D magneto-micropolar
fluid equations (1.1) in terms of one directional derivative of the velocity u or of the pressure π and the magnetic field b by adapting the method of [27]. Without loss of generality, we set the viscous coefficients .

We now state our main results as follows.

Theorem 1.1Assume thatwith. Letbe the corresponding local smooth solution to the magneto-micropolar fluid equations (1.1) onfor some. If the velocityusatisfies

(1.4)

thencan be extended beyondT.

Note that when , and thus the corresponding assumption in (1.4) should be understood as .

Remark 1.1 Theorem 1.1 improves the regularity criterion in [32] (see (1.3)) in the sense that it depends only on one directional derivative of the
velocity u.

Theorem 1.2Assume thatwith. Letbe the corresponding local smooth solution to the magneto-micropolar fluid equations (1.1) onfor some. If the pressureπand the magnetic fieldbsatisfy

(1.5)

thencan be extended beyondT.

Remark 1.2 When , we also obtain a new regularity criterion for the micropolar equations determined
by one direction derivative of the pressure π alone.

We shall prove our results in the next section. For simplicity, we denote by the norm and by the inner product throughout the paper. The letter C denotes an inessential constant which might vary from line to line, but does not
depend on particular solutions or functions.

2 Proof of the main results

In this section, we give the proof of Theorem 1.1 and Theorem 1.2. The following
lemma plays an important role in our arguments. Its proof can be found in [37] or [27].

Lemma 2.1Let the parameters, , andrsatisfy

and suppose that (). Then there exists a constantsuch that

In particular, whenand, there exists a constantsuch that

for anyφsatisfyingand.

Proof of Theorem 1.1 Observe that for any with , there exists a unique local smooth solution to equations (1.1) (see [2]). Let be the maximum existence time. To prove Theorem 1.1, it is sufficient to show that
the assumption (1.4) implies . Indeed, we shall prove that under the condition (1.4), there exists a constant such that

(2.1)

which implies that T is not the maximum existence time and thus the solution can be extended beyond T by the standard arguments of continuation of local solutions.

Firstly, we derive the energy inequality. For this purpose, we take the inner product of u, w and b with equations (1.1), respectively, sum the resulting equations and then integrate
by parts to obtain

where we used in the first equality and Hölder’s inequality in the last inequality. Thus,

It follows from Gronwall’s inequality that

(2.2)

Now we split the proof of the estimates (2.1) into two steps.

Step 1: Estimates for .

To this end, differentiating the first three equations in (1.1) with respect to , taking the inner product of , and with the resulting equations, respectively, and then performing a space integration
by parts, we get

where we used the facts

by . Noticing that

by , we can sum the above equations to obtain

We now estimate the above terms one by one. To bound , we first integrate by parts and then apply Hölder’s inequality to obtain

(2.3)

It follows from the Gagliardo-Nirenberg inequality that

and from Lemma 2.1 that

Substituting these two estimates into (2.3) and then using Young’s inequality, we
see that for

(2.4)

and that for

(2.5)

For , by Hölder’s inequality, the Gagliardo-Nirenberg inequality and Young’s inequality,
we have for

(2.6)

and for

(2.7)

Applying similar procedure to and , we have for

(2.8)

and

(2.9)

and for

(2.10)

For the term , by using Hölder’s inequality and Young’s inequality, it can be bounded as follows:

(2.11)

Finally, we can follow the steps as in the bound of to estimate . Precisely, by integrations by parts and Hölder’s inequality, we have

Then the Gagliardo-Nirenberg inequality, Lemma 2.1 and Young’s inequality yield that
for

(2.12)

and for

(2.13)

Combining the estimates (2.4)-(2.12), we see that for

and that for

Thus, Gronwall’s inequality together with the energy inequality (2.2) and the assumption
(1.4) implies that for

with , and

Then

(2.14)

which is the desired estimates.

Step 2: Estimates for .

For this purpose, taking the inner product of Δu, Δw and Δb with the first three equations in (1.1), respectively, and then performing a space
integration by parts, we have

Noticing , we sum the above equations and integrate by parts to obtain

(2.15)

By using the interpolation inequality and taking in Lemma 2.1, we have

where . Then Young’s inequality yields

Similarly,

Substituting the above two estimates into (2.15), we have

By using Gronwall’s inequality, the energy inequality (2.2) and the estimate (2.14),
we conclude that

for any , which implies that the desired estimates (2.1) hold and thus the solution can be extended beyond T. □

Now we turn our attention to proving Theorem 1.2. We will first transform equations
(1.1) into a symmetric form.

Proof of Theorem 1.2 Following from Serrin type criteria (1.2) with and on the 3D magneto-micropolar fluid equations (1.1), it is sufficient to prove that

(2.16)

To do this, we set

and then equations (1.1) are converted to the following symmetric form:

(2.17)

Firstly, taking the inner product of , w and with the above equations, respectively, and integrating by parts, we can obtain the
energy estimates similar to (2.2).

Next we take the inner product of , and with the first three equations in (2.17), respectively, and then integrate by parts
to obtain

We now bound the above terms one by one. For , we have

It follows from the integration by parts, we see

Similarly, we have

and

The process for estimating is more subtle. It follows from Hölder’s inequality and Lemma 2.1 that

To estimate the term involving , we take the divergence of the first equation of (2.17) and find

by . Then the Calderón-Zygmund inequality, Hölder’s inequality and the interpolation
inequality imply that

Similarly, we have

If , combining the above two estimates, we see

The case can be similarly dealt with.

Summarily, we conclude that

Thus, Gronwall’s inequality together with the assumption (1.5) and the energy estimates
gives the desired estimates (2.12) and thus the solution can be extended beyond T. □

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

ZX wrote the first draft and HY corrected and improved it. Both authors read and approved
the final draft.

Acknowledgements

The authors would like to thank the referees for their valuable comments and remarks.
This work was partially supported by the NNSF of China (No. 11101068), the Sichuan
Youth Science & Technology Foundation (No. 2011JQ0003), the SRF for ROCS, SEM, and
the Fundamental Research Funds for the Central Universities (ZYGX2009X019).